Dark matter and dark energy. For quite sometime I struggled with what the two were. At first I thought that the two were one and the same thing, I mean they both had the same work 'dark' in them. But it is unnatural for Physicists to use synonyms to describe similar concepts(take velocity and speed - both describe how fast an object is moving but speed is a scalar quantity while velocity is a vector quantity). So I did what any normal person in my situations would do and turned to my good ol' pal, the internet because it never lies, right? After some time it become clear that, while yes we do know that the two exist, no one know the how, when or why they do. But through numerous experiments, we know more about what dark matter is not.
Dark matter
While questions about dark matter have been the talk of the Physics community in the 21 century, this vaguely understood subject matter has been under scrutiny since the 19 century when Lord kelvin a Physicist concluded in one of his lectures that "many of our stars, perhaps a great majority of them, maybe dark bodies", suggesting that there might be do not interact with light - they do not reflect or radiate light - hence difficult to observe with normal astronomical instruments. Later on after an astronomer Fritz Zwicky, after studying the Coma galaxy cluster determined that it did not have enough visible matter to hold it together. He found out that the 800 galaxies he studied should have had a velocity dispersion of 80 km/s but discovered that the actual value was closer to 1000 km/s. With this speed the stars should have escaped they mutual gravitational attraction, but because they hadn't meant that there was more mass than could be accounted for with visible matter.
The modern concept of dark matter was drawn from this idea - something that does not radiate light, whose existence can only be confirmed by observing the effects it has on visible matter.
The first substantive piece evidence for dark matter came from astronomer Vera Rubin using a spectrometer developed by her collaborator, astronomer Kent Ford. Because the core region of a spiral galaxy has the highest concentration of visible stars, astronomers assumed that most of the mass and hence gravity of a galaxy would also be concentrated towards the centre and this would mean that the farther away a star is away from the centre, the slower its orbital speed is expected to be. Same way outer planets in our solar system move slower than the inner planets in our solar system. What they noticed was that the stars on a galaxy's outskirts orbit just as fast or faster than the stars closer in. Again, the visible matter did not have enough gravity to hold these fast moving stars together. This was more evidence that some 'invisible stuff' was holding these galaxies together.
Dark energy
According to The Big Bang Theory, all the past and current matter came into existence at the same time, around 13.8-ish billion years ago. In the beginning all matter was condensed up into a very small ball with infinite density and intense heat called a singularity. Suddenly the universe began to expand and inflate and this led to the universe as we know it know. Physicist had initially assumed that the attractive force of gravity would slow down the expansion of the universe over time. Perhaps this would take out the universe when it collapsed back on itself in a big crunch. But in the 1990s when two teams of astronomers tried to measure the rate of deceleration, they found out that the expansion was actually speeding up. This 'force' behind this accelerated expansion is widely believed to be 'dark energy' and unlike dark matter scientists have no plausible explanation far it. What's more is that the 'force' seems to be growing stronger as the universe expands.
Einstein included a cosmological constant to his relativity theory - and letter regretted - to keep the universe from collapsing on itself and thanks to him scientists can use it to express dark matter. (Overly simplified)